Resistive transition, magnetoresistance, and anisotropy in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ single-crystal thin films

The temperature dependence of the resistivity ρ(T) in the vicinity of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ has been measured for thin-film single crystals of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ as a function of an applied magnetic field, both parallel and perpendicular to the c axis. The resistive transition exhibits characteristic behavior, depending on the Sr concentration x, when a field is applied. For small x the application of a field causes a significant broadening of the resistive transition—a behavior thought to be typical of high-${\mathit{T}}_{\mathit{c}}$ cuprate superconductors—while for x exceeding 0.15, it simply causes a parallel shift of the curves to lower temperatures. From the analysis of the magnetoresistance above ${\mathit{T}}_{\mathit{c}}$ based on the theory of field-dependent fluctuation conductivity for layered materials, it is shown that the behavior of the field-induced resistive transition can be ascribed to the flux motion, especially when H⊥c. The analysis also shows that the out-of-plane Ginzburg-Landau coherence length ${\xi }_{\mathit{c}}$(0) increases systematically with x from 0.55 Å at x=0.08 to 3 Å at x=0.3, while the in-plane Ginsburg-Landau coherence length ${\xi }_{\mathit{a}\mathit{b}}$(0) is almost constant and ${\xi }_{\mathit{a}\mathit{b}}$(0)=31–33 Å, irrespective of the value of x as long as ${\mathit{T}}_{\mathit{c}}$ remains high. The anisotropy ${\xi }_{\mathit{a}\mathit{b}}$(0)/${\xi }_{\mathit{c}}$(0) estimated from these results exceeds 50 at low Sr concentrations and is still larger than 15 even for x>0.1.